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Network Working Group                                         J. LucianiRequest for Comments: 2332                                  Bay NetworksCategory: Standards Track                                        D. Katz                                                           cisco Systems                                                           D. Piscitello                                                   Core Competence, Inc.                                                                 B. Cole                                                        Juniper Networks                                                            N. Doraswamy                                                            Bay Networks                                                              April 1998NBMA Next Hop Resolution Protocol (NHRP)Status of this Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (1998).  All Rights Reserved.Abstract   This document describes the NBMA Next Hop Resolution Protocol (NHRP).   NHRP can be used by a source station (host or router) connected to a   Non-Broadcast, Multi-Access (NBMA) subnetwork to determine the   internetworking layer address and NBMA subnetwork addresses of the   "NBMA next hop" towards a destination station.  If the destination is   connected to the NBMA subnetwork, then the NBMA next hop is the   destination station itself.  Otherwise, the NBMA next hop is the   egress router from the NBMA subnetwork that is "nearest" to the   destination station.  NHRP is intended for use in a multiprotocol   internetworking layer environment over NBMA subnetworks.   Note that while this protocol was developed for use with NBMA   subnetworks, it is possible, if not likely, that it will be applied   to BMA subnetworks as well.  However, this usage of NHRP is for   further study.   This document is intended to be a functional superset of the NBMA   Address Resolution Protocol (NARP) documented in [1].Luciani, et. al.            Standards Track                     [Page 1]

RFC 2332                       NBMA NHRP                      April 1998   Operation of NHRP as a means of establishing a transit path across an   NBMA subnetwork between two routers will be addressed in a separate   document (see [13]).1. Introduction   The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this   document, are to be interpreted as described in [15].   The NBMA Next Hop Resolution Protocol (NHRP) allows a source station   (a host or router), wishing to communicate over a Non-Broadcast,   Multi-Access (NBMA) subnetwork, to determine the internetworking   layer addresses and NBMA addresses of suitable "NBMA next hops"   toward a destination station.  A subnetwork can be non-broadcast   either because it technically doesn't support broadcasting (e.g., an   X.25 subnetwork) or because broadcasting is not feasible for one   reason or another (e.g., an SMDS multicast group or an extended   Ethernet would be too large).  If the destination is connected to the   NBMA subnetwork, then the NBMA next hop is the destination station   itself.  Otherwise, the NBMA next hop is the egress router from the   NBMA subnetwork that is "nearest" to the destination station.   One way to model an NBMA network is by using the notion of logically   independent IP subnets (LISs). LISs, as defined in [3] and [4], have   the following properties:      1)  All members of a LIS have the same IP network/subnet number          and address mask.      2)  All members of a LIS are directly connected to the same          NBMA subnetwork.      3)  All hosts and routers outside of the LIS are accessed via          a router.      4)  All members of a LIS access each other directly (without          routers).   Address resolution as described in [3] and [4] only resolves the next   hop address if the destination station is a member of the same LIS as   the source station; otherwise, the source station must forward   packets to a router that is a member of multiple LIS's.  In multi-LISLuciani, et. al.            Standards Track                     [Page 2]

RFC 2332                       NBMA NHRP                      April 1998   configurations, hop-by-hop address resolution may not be sufficient   to resolve the "NBMA next hop" toward the destination station, and IP   packets may have multiple IP hops through the NBMA subnetwork.   Another way to model NBMA is by using the notion of Local Address   Groups (LAGs) [10]. The essential difference between the LIS and the   LAG models is that while with the LIS model the outcome of the   "local/remote" forwarding decision is driven purely by addressing   information, with the LAG model the outcome of this decision is   decoupled from the addressing information and is coupled with the   Quality of Service and/or traffic characteristics.  With the LAG   model any two entities on a common NBMA network could establish a   direct communication with each other, irrespective of the entities'   addresses.   Support for the LAG model assumes the existence of a mechanism that   allows any entity (i.e., host or router) connected to an NBMA network   to resolve an internetworking layer address to an NBMA address for   any other entity connected to the same NBMA network.  This resolution   would take place regardless of the address assignments to these   entities. Within the parameters described in this document, NHRP   describes such a mechanism.  For example, when the internetworking   layer address is of type IP, once the NBMA next hop has been   resolved, the source may either start sending IP packets to the   destination (in a connectionless NBMA subnetwork such as SMDS) or may   first establish a connection to the destination with the desired   bandwidth (in a connection-oriented NBMA subnetwork such as ATM).   Use of NHRP may be sufficient for hosts doing address resolution when   those hosts are directly connected to an NBMA subnetwork, allowing   for straightforward implementations in NBMA stations. NHRP also has   the capability of determining the egress point from an NBMA   subnetwork when the destination is not directly connected to the NBMA   subnetwork and the identity of the egress router is not learned by   other methods (such as routing protocols).  Optional extensions to   NHRP provide additional robustness and diagnosability.   Address resolution techniques such as those described in [3] and [4]   may be in use when NHRP is deployed.  ARP servers and services over   NBMA subnetworks may be required to support hosts that are not   capable of dealing with any model for communication other than the   LIS model, and deployed hosts may not implement NHRP but may continue   to support ARP variants such as those described in [3] and [4].  NHRP   is intended to reduce or eliminate the extra router hops required by   the LIS model, and can be deployed in a non-interfering manner with   existing ARP services [14].Luciani, et. al.            Standards Track                     [Page 3]

RFC 2332                       NBMA NHRP                      April 1998   The operation of NHRP to establish transit paths across NBMA   subnetworks between two routers requires additional mechanisms to   avoid stable routing loops, and will be described in a separate   document (see [13]).2. Overview2.1 Terminology   The term "network" is highly overloaded, and is especially confusing   in the context of NHRP.  We use the following terms:     Internetwork layer--the media-independent layer (IP in the case of     TCP/IP networks).     Subnetwork layer--the media-dependent layer underlying the     internetwork layer, including the NBMA technology (ATM, X.25, SMDS,     etc.)     The term "server", unless explicitly stated to the contrary, refers     to a Next Hop Server (NHS).  An NHS is an entity performing the     Next Hop Resolution Protocol service within the NBMA cloud.  An NHS     is always tightly coupled with a routing entity (router, route     server or edge device) although the converse is not yet guaranteed     until ubiquitous deployment of this functionality occurs.  Note     that the presence of intermediate routers that are not coupled with     an NHS entity may preclude the use of NHRP when source and     destination stations on different sides of such routers and thus     such routers may partition NHRP reachability within an NBMA     network.     The term "client", unless explicitly stated to the contrary, refers     to a Next Hop Resolution Protocol client (NHC).  An NHC is an     entity which initiates NHRP requests of various types in order to     obtain access to the NHRP service.     The term "station" generally refers to a host or router which     contains an NHRP entity.  Occasionally, the term station will     describe a "user" of the NHRP client or service functionality; the     difference in usage is largely semantic.2.2 Protocol Overview   In this section, we briefly describe how a source S (which   potentially can be either a router or a host) uses NHRP to determine   the "NBMA next hop" to destination D.Luciani, et. al.            Standards Track                     [Page 4]

RFC 2332                       NBMA NHRP                      April 1998   For administrative and policy reasons, a physical NBMA subnetwork may   be partitioned into several, disjoint "Logical NBMA subnetworks".  A   Logical NBMA subnetwork is defined as a collection of hosts and   routers that share unfiltered subnetwork connectivity over an NBMA   subnetwork.  "Unfiltered subnetwork connectivity" refers to the   absence of closed user groups, address screening or similar features   that may be used to prevent direct communication between stations   connected to the same NBMA subnetwork.  (Hereafter, unless otherwise   specified, we use the term "NBMA subnetwork" to mean *logical* NBMA   subnetwork.)   Placed within the NBMA subnetwork are one or more entities that   implement the NHRP protocol.  Such stations which are capable of   answering NHRP Resolution Requests are known as "Next Hop Servers"   (NHSs).  Each NHS serves a set of destination hosts, which may or may   not be directly connected to the NBMA subnetwork.  NHSs cooperatively   resolve the NBMA next hop within their logical NBMA subnetwork.  In   addition to NHRP, NHSs may support "classical" ARP service; however,   this will be the subject of a separate document [14].   An NHS maintains a cache which contains protocol layer address to   NBMA subnetwork layer address resolution information.  This cache can   be constructed from information obtained from NHRP Register packets   (seeSection 5.2.3 and 5.2.4), from NHRP Resolution Request/Reply   packets, or through mechanisms outside the scope of this document   (examples of such mechanisms might include ARP[3] and pre-configured   tables).Section 6.2 further describes cache management issues.   For a station within a given LIS to avoid providing NHS   functionality, there must be one or more NHSs within the NBMA   subnetwork which are providing authoritative address resolution   information on its behalf.  Such an NHS is said to be "serving" the   station.  A station on a LIS that lacks NHS functionality and is a   client of the NHRP service is known as NHRP Client or just NHCs.  If   a serving NHS is to be able to supply the address resolution   information for an NHC then NHSs must exist at each hop along all   routed paths between the NHC making the resolution request and the   destination NHC.  The last NHRP entity along the routed path is the   serving NHS; that is, NHRP Resolution Requests are not forwarded to   destination NHCs but rather are processed by the serving NHS.   An NHC also maintains a cache of protocol address to NBMA address   resolution information.  This cache is populated through information   obtained from NHRP Resolution Reply packets, from manual   configuration, or through mechanisms outside the scope of this   document.Luciani, et. al.            Standards Track                     [Page 5]

RFC 2332                       NBMA NHRP                      April 1998   The protocol proceeds as follows.  An event occurs triggering station   S to want to resolve the NBMA address of a path to D.  This is most   likely to be when a data packet addressed to station D is to be   emitted from station S (either because station S is a host, or   station S is a transit router), but the address resolution could also   be triggered by other means (a routing protocol update packet, for   example). Station S first determines the next hop to station D   through normal routing processes (for a host, the next hop may simply   be the default router; for routers, this is the "next hop" to the   destination internetwork layer address).  If the destination's   address resolution information is already available in S's cache then   that information is used to forward the packet.  Otherwise, if the   next hop is reachable through one of its NBMA interfaces, S   constructs an NHRP Resolution Request packet (seeSection 5.2.1)   containing station D's internetwork layer address as the (target)   destination address, S's own internetwork layer address as the source   address (Next Hop Resolution Request initiator), and station S's NBMA   addressing information.  Station S may also indicate that it prefers   an authoritative NHRP Resolution Reply (i.e., station S only wishes   to receive an NHRP Resolution Reply from an NHS serving the   destination NHC). Station S emits the NHRP Resolution Request packet   towards the destination.   If the NHRP Resolution Request is triggered by a data packet then S   may, while awaiting an NHRP Resolution Reply, choose to dispose of   the data packet in one of the following ways:     (a)  Drop the packet     (b)  Retain the packet until the NHRP Resolution Reply arrives          and a more optimal path is available     (c)  Forward the packet along the routed path toward D   The choice of which of the above to perform is a local policy matter,   though option (c) is the recommended default, since it may allow data   to flow to the destination while the NBMA address is being resolved.   Note that an NHRP Resolution Request for a given destination MUST NOT   be triggered on every packet.   When the NHS receives an NHRP Resolution Request, a check is made to   see if it serves station D.  If the NHS does not serve D, the NHS   forwards the NHRP Resolution Request to another NHS.  Mechanisms for   determining how to forward the NHRP Resolution Request are discussed   inSection 3.   If this NHS serves D, the NHS resolves station D's NBMA address   information, and generates a positive NHRP Resolution Reply on D's   behalf.  NHRP Resolution Replies in this scenario are always marked   as "authoritative".  The NHRP Resolution Reply packet contains theLuciani, et. al.            Standards Track                     [Page 6]

RFC 2332                       NBMA NHRP                      April 1998   address resolution information for station D which is to be sent back   to S.  Note that if station D is not on the NBMA subnetwork, the next   hop internetwork layer address will be that of the egress router   through which packets for station D are forwarded.   A transit NHS receiving an NHRP Resolution Reply may cache the   address resolution information contained therein.  To a subsequent   NHRP Resolution Request, this NHS may respond with the cached, "non-   authoritative" address resolution information if the NHS is permitted   to do so (see Sections5.2.2 and6.2 for more information on non-   authoritative versus authoritative NHRP Resolution Replies).  Non-   authoritative NHRP Resolution Replies are distinguished from   authoritative NHRP Resolution Replies so that if a communication   attempt based on non-authoritative information fails, a source   station can choose to send an authoritative NHRP Resolution Request.   NHSs MUST NOT respond to authoritative NHRP Resolution Requests with   cached information.   If the determination is made that no NHS in the NBMA subnetwork can   reply to the NHRP Resolution Request for D then a negative NHRP   Resolution Reply (NAK) is returned.  This occurs when (a) no next-hop   resolution information is available for station D from any NHS, or   (b) an NHS is unable to forward the NHRP Resolution Request (e.g.,   connectivity is lost).   NHRP Registration Requests, NHRP Purge Requests, NHRP Purge Replies,   and NHRP Error Indications follow a routed path in the same fashion   that NHRP Resolution Requests and NHRP Resolution Replies do.   Specifically, "requests" and "indications" follow the routed path   from Source Protocol Address (which is the address of the station   initiating the communication) to the Destination Protocol Address.   "Replies", on the other hand, follow the routed path from the   Destination Protocol Address back to the Source Protocol Address with   the following exceptions: in the case of a NHRP Registration Reply   and in the case of an NHC initiated NHRP Purge Request, the packet is   always returned via a direct VC (see Sections5.2.4 and5.2.5); if   one does not exists then one MUST be created.   NHRP Requests and NHRP Replies do NOT cross the borders of a NBMA   subnetwork however further study is being done in this area (seeSection 7).   Thus, the internetwork layer data traffic out of and   into an NBMA subnetwork always traverses an internetwork layer router   at its border.   NHRP optionally provides a mechanism to send a NHRP Resolution Reply   which contains aggregated address resolution information. For   example, suppose that router X is the next hop from station S to   station D and that X is an egress router for all stations sharing anLuciani, et. al.            Standards Track                     [Page 7]

RFC 2332                       NBMA NHRP                      April 1998   internetwork layer address prefix with station D.  When an NHRP   Resolution Reply is generated in response to a NHRP Resolution   Request, the responder may augment the internetwork layer address of   station D with a prefix length (seeSection 5.2.0.1).  A subsequent   (non-authoritative) NHRP Resolution Request for some destination that   shares an internetwork layer address prefix (for the number of bits   specified in the prefix length) with D may be satisfied with this   cached information.  Seesection 6.2 regarding caching issues.   To dynamically detect subnetwork-layer filtering in NBMA subnetworks   (e.g., X.25 closed user group facility, or SMDS address screens), to   trace the routed path that an NHRP packet takes, or to provide loop   detection and diagnostic capabilities, a "Route Record" may be   included in NHRP packets (see Sections5.3.2 and5.3.3).  The Route   Record extensions are the NHRP Forward Transit NHS Record Extension   and the NHRP Reverse Transit NHS Record Extension.  They contain the   internetwork (and subnetwork layer) addresses of all intermediate   NHSs between source and destination and between destination and   source respectively.  When a source station is unable to communicate   with the responder (e.g., an attempt to open an SVC fails), it may   attempt to do so successively with other subnetwork layer addresses   in the NHRP Forward Transit NHS Record Extension until it succeeds   (if authentication policy permits such action).  This approach can   find a suitable egress point in the presence of subnetwork-layer   filtering (which may be source/destination sensitive, for instance,   without necessarily creating separate logical NBMA subnetworks) or   subnetwork-layer congestion (especially in connection-oriented   media).3. Deployment   NHRP Resolution Requests traverse one or more hops within an NBMA   subnetwork before reaching the station that is expected to generate a   response.  Each station, including the source station, chooses a   neighboring NHS to which it will forward the NHRP Resolution Request.   The NHS selection procedure typically involves applying a destination   protocol layer address to the protocol layer routing table which   causes a routing decision to be returned.  This routing decision is   then used to forward the NHRP Resolution Request to the downstream   NHS. The destination protocol layer address previously mentioned is   carried within the NHRP Resolution Request packet.  Note that even   though a protocol layer address was used to acquire a routing   decision, NHRP packets are not encapsulated within a protocol layer   header but rather are carried at the NBMA layer using the   encapsulation described inSection 5.Luciani, et. al.            Standards Track                     [Page 8]

RFC 2332                       NBMA NHRP                      April 1998   Each NHS/router examines the NHRP Resolution Request packet on its   way toward the destination.  Each NHS which the NHRP packet traverses   on the way to the packet's destination might modify the packet (e.g.,   updating the Forward Record extension).  Ignoring error situations,   the NHRP Resolution Request eventually arrives at a station that is   to generate an NHRP Resolution Reply.  This responding station   "serves" the destination.  The responding station generates an NHRP   Resolution Reply using the source protocol address from within the   NHRP packet to determine where the NHRP Resolution Reply should be   sent.   Rather than use routing to determine the next hop for an NHRP packet,   an NHS may use other applicable means (such as static configuration   information ) in order to determine to which neighboring NHSs to   forward the NHRP Resolution Request packet as long as such other   means would not cause the NHRP packet to arrive at an NHS which is   not along the routed path.  The use of static configuration   information for this purpose is beyond the scope of this document.   The NHS serving a particular destination must lie along the routed   path to that destination.  In practice, this means that all egress   routers must double as NHSs serving the destinations beyond them, and   that hosts on the NBMA subnetwork are served by routers that double   as NHSs.  Also, this implies that forwarding of NHRP packets within   an NBMA subnetwork requires a contiguous deployment of NHRP capable   routers.  It is important that, in a given LIS/LAG which is using   NHRP, all NHSs within the LIS/LAG have at least some portion of their   resolution databases synchronized so that a packet arriving at one   router/NHS in a given LIS/LAG will be forwarded in the same fashion   as a packet arriving at a different router/NHS for the given LIS/LAG.   One method, among others, is to use the Server Cache Synchronization   Protocol (SCSP) [12].  It is RECOMMENDED that SCSP be the method used   when a LIS/LAG contains two or more router/NHSs.   During migration to NHRP, it cannot be expected that all routers   within the NBMA subnetwork are NHRP capable.  Thus, NHRP traffic   which would otherwise need to be forwarded through such routers can   be expected to be dropped due to the NHRP packet not being   recognized.  In this case, NHRP will be unable to establish any   transit paths whose discovery requires the traversal of the non-NHRP   speaking routers.  If the client has tried and failed to acquire a   cut through path then the client should use the network layer routed   path as a default.   If an NBMA technology offers a group, an anycast, or a multicast   addressing feature then the NHC may be configured with such an   address (appropriate to the routing realm it participates in) which   would be assigned to all NHS serving that routing realm.  ThisLuciani, et. al.            Standards Track                     [Page 9]

RFC 2332                       NBMA NHRP                      April 1998   address can then be used for establishing an initial connection to an   NHS to transmit a registration request.  This address may not be used   for sending NHRP requests.  The resulting VC may be used for NHRP   requests if and only if the registration response is received over   that VC, thereby indicating that one happens to have anycast   connected to an NHS serving the LIS/LAG.  In the case of non-   connection oriented networks, or of multicast (rather than anycast)   addresses, the addres MUST NOT be used for sending NHRP resolution   requests.   When an NHS "serves" an NHC, the NHS MUST send NHRP messages destined   for the NHC directly to the NHC.  That is, the NHRP message MUST NOT   transit through any NHS which is not serving the NHC when the NHRP   message is currently at an NHS which does serve the NHC (this, of   course, assumes the NHRP message is destined for the NHC).  Further,   an NHS which serves an NHC SHOULD have a direct NBMA level connection   to that NHC (seeSection 5.2.3 and 5.2.4 for examples).   With the exception of NHRP Registration Requests (seeSection 5.2.3   and 5.2.4 for details of the NHRP Registration Request case), an NHC   MUST send NHRP messages over a direct NBMA level connection between   the serving NHS and the served NHC.   It may not be desirable to maintain semi-permanent NBMA level   connectivity between the NHC and the NHS.   In this case, when NBMA   level connectivity is initially setup between the NHS and the NHC (as   described inSection 5.2.4), the NBMA address of the NHS should be   obtained through the NBMA level signaling technology.  This address   should be stored for future use in setting up subsequent NBMA level   connections.  A somewhat more information rich technique to obtain   the address information (and more) of the serving NHS would be for   the NHC to include the Responder Address extension (seeSection5.3.1) in the NHRP Registration Request and to store the information   returned to the NHC in the Responder Address extension which is   subsequently included in the NHRP Registration Reply.  Note also   that, in practice, a client's default router should also be its NHS;   thus a client may be able to know the NBMA address of its NHS from   the configuration which was already required for the client to be   able to communicate.  Further, as mentioned inSection 4, NHCs may be   configured with the addressing information of one or more NHSs.4. Configuration   Next Hop Clients     An NHC connected to an NBMA subnetwork MAY be configured with the     Protocol address(es) and NBMA address(es) of its NHS(s).  The     NHS(s) will likely also represent the NHC's default or peerLuciani, et. al.            Standards Track                    [Page 10]

RFC 2332                       NBMA NHRP                      April 1998     routers, so their NBMA addresses may be obtained from the NHC's     existing configuration.  If the NHC is attached to several     subnetworks (including logical NBMA subnetworks), the NHC should     also be configured to receive routing information from its NHS(s)     and peer routers so that it can determine which internetwork layer     networks are reachable through which subnetworks.   Next Hop Servers     An NHS is configured with knowledge of its own internetwork layer     and NBMA addresses.  An NHS MAY also be configured with a set of     internetwork layer address prefixes that correspond to the     internetwork layer addresses of the stations it serves. The NBMA     addresses of the stations served by the NHS may be learned via NHRP     Registration packets.     If a served NHC is attached to several subnetworks, the     router/route-server coresident with the serving NHS may also need     to be configured to advertise routing information to such NHCs.     If an NHS acts as an egress router for stations connected to other     subnetworks than the NBMA subnetwork, the NHS must, in addition to     the above, be configured to exchange routing information between     the NBMA subnetwork and these other subnetworks.     In all cases, routing information is exchanged using conventional     intra-domain and/or inter-domain routing protocols.5. NHRP Packet Formats   This section describes the format of NHRP packets.  In the following,   unless otherwise stated explicitly, the unqualified term "request"   refers generically to any of the NHRP packet types which are   "requests".  Further, unless otherwise stated explicitly, the   unqualified term "reply" refers generically to any of the NHRP packet   types which are "replies".   An NHRP packet consists of a Fixed Part, a Mandatory Part, and an   Extensions Part.  The Fixed Part is common to all NHRP packet types.   The Mandatory Part MUST be present, but varies depending on packet   type.  The Extensions Part also varies depending on packet type, and   need not be present.   The length of the Fixed Part is fixed at 20 octets.  The length of   the Mandatory Part is determined by the contents of the extensions   offset field (ar$extoff).  If ar$extoff=0x0 then the mandatory part   length is equal to total packet length (ar$pktsz) minus 20 otherwise   the mandatory part length is equal to ar$extoff minus 20.  The lengthLuciani, et. al.            Standards Track                    [Page 11]

RFC 2332                       NBMA NHRP                      April 1998   of the Extensions Part is implied by ar$pktsz minus ar$extoff.  NHSs   may increase the size of an NHRP packet as a result of extension   processing, but not beyond the offered maximum packet size of the   NBMA network.   NHRP packets are actually members of a wider class of address mapping   and management protocols being developed by the IETF. A specific   encapsulation, based on the native formats used on the particular   NBMA network over which NHRP is carried, indicates the generic IETF   mapping and management protocol. For example, SMDS networks always   use LLC/SNAP encapsulation at the NBMA layer [4], and an NHRP packet   is preceded by the following LLC/SNAP encapsulation:   [0xAA-AA-03] [0x00-00-5E] [0x00-03]   The first three octets are LLC, indicating that SNAP follows.  The   SNAP OUI portion is the IANA's OUI, and the SNAP PID portion   identifies the mapping and management protocol. A field in the Fixed   Header following the encapsulation indicates that it is NHRP.   ATM uses either LLC/SNAP encapsulation of each packet (including   NHRP), or uses no encapsulation on VCs dedicated to a single protocol   (see [7]).  Frame Relay and X.25 both use NLPID/SNAP encapsulation or   identification of NHRP, using a NLPID of 0x0080 and the same SNAP   contents as above (see [8], [9]).   Fields marked "unused" MUST be set to zero on transmission, and   ignored on receipt.   Most packet types (ar$op.type) have both internetwork layer   protocol-independent fields and protocol-specific fields. The   protocol type/snap fields (ar$pro.type/snap) qualify the format of   the protocol-specific fields.5.1 NHRP Fixed Header   The Fixed Part of the NHRP packet contains those elements of the NHRP   packet which are always present and do not vary in size with the type   of packet.Luciani, et. al.            Standards Track                    [Page 12]

RFC 2332                       NBMA NHRP                      April 1998    0                   1                   2                   3    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |            ar$afn             |          ar$pro.type          |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                          ar$pro.snap                          |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |  ar$pro.snap  |   ar$hopcnt   |            ar$pktsz           |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |           ar$chksum           |            ar$extoff          |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   | ar$op.version |   ar$op.type  |    ar$shtl    |    ar$sstl    |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   ar$afn     Defines the type of "link layer" addresses being carried.  This     number is taken from the 'address family number' list specified in     [6].  This field has implications to the coding of ar$shtl and     ar$sstl as described below.   ar$pro.type     field is a 16 bit unsigned integer representing the following     number space:       0x0000 to 0x00FF  Protocols defined by the equivalent NLPIDs.       0x0100 to 0x03FF  Reserved for future use by the IETF.       0x0400 to 0x04FF  Allocated for use by the ATM Forum.       0x0500 to 0x05FF  Experimental/Local use.       0x0600 to 0xFFFF  Protocols defined by the equivalent Ethertypes.     (based on the observations that valid Ethertypes are never smaller     than 0x600, and NLPIDs never larger than 0xFF.)   ar$pro.snap     When ar$pro.type has a value of 0x0080, a SNAP encoded extension is     being used to encode the protocol type. This snap extension is     placed in the ar$pro.snap field.  This is termed the 'long form'     protocol ID. If ar$pro != 0x0080 then the ar$pro.snap field MUST be     zero on transmit and ignored on receive. The ar$pro.type field     itself identifies the protocol being referred to. This is termed     the 'short form' protocol ID.     In all cases, where a protocol has an assigned number in the     ar$pro.type space (excluding 0x0080) the short form MUST be used     when transmitting NHRP messages; i.e., if Ethertype or NLPID     codings exist then they are used on transmit rather than theLuciani, et. al.            Standards Track                    [Page 13]

RFC 2332                       NBMA NHRP                      April 1998     ethertype.   If both Ethertype and NLPID codings exist then when     transmitting NHRP messages, the Ethertype coding MUST be used (this     is consistent withRFC 1483 coding).  So, for example, the     following codings exist for IP:       SNAP:      ar$pro.type = 0x00-80, ar$pro.snap = 0x00-00-00-08-00       NLPID:     ar$pro.type = 0x00-CC, ar$pro.snap = 0x00-00-00-00-00       Ethertype: ar$pro.type = 0x08-00, ar$pro.snap = 0x00-00-00-00-00     and thus, since the Ethertype coding exists, it is used in     preference.   ar$hopcnt     The Hop count indicates the maximum number of NHSs that an NHRP     packet is allowed to traverse before being discarded.  This field     is used in a similar fashion to the way that a TTL is used in an IP     packet and should be set accordingly.  Each NHS decrements the TTL     as the NHRP packet transits the NHS on the way to the next hop     along the routed path to the destination.  If an NHS receives an     NHRP packet which it would normally forward to a next hop and that     packet contains an ar$hopcnt set to zero then the NHS sends an     error indication message back to the source protocol address     stating that the hop count has been exceeded (seeSection 5.2.7)     and the NHS drops the packet in error;  however, an error     indication is never sent as a result of receiving an error     indication.  When a responding NHS replies to an NHRP request, that     NHS places a value in ar$hopcnt as if it were sending a request of     its own.   ar$pktsz     The total length of the NHRP packet, in octets (excluding link     layer encapsulation).   ar$chksum     The standard IP checksum over the entire NHRP packet starting at     the fixed header.  If the packet is an odd number of bytes in     length then this calculation is performed as if a byte set to 0x00     is appended to the end of the packet.   ar$extoff     This field identifies the existence and location of NHRP     extensions.  If this field is 0 then no extensions exist otherwise     this field represents the offset from the beginning of the NHRP     packet (i.e., starting from the ar$afn field) of the first     extension.Luciani, et. al.            Standards Track                    [Page 14]

RFC 2332                       NBMA NHRP                      April 1998   ar$op.version     This field indicates what version of generic address mapping and     management protocol is represented by this message.       0               MARS protocol [11].       1               NHRP as defined in this document.       0x02 - 0xEF     Reserved for future use by the IETF.       0xF0 - 0xFE     Allocated for use by the ATM Forum.       0xFF            Experimental/Local use.   ar$op.type     When ar$op.version == 1, this is the NHRP packet type: NHRP     Resolution Request(1), NHRP Resolution Reply(2), NHRP Registration     Request(3), NHRP Registration Reply(4), NHRP Purge Request(5), NHRP     Purge Reply(6), or NHRP Error Indication(7).  Use of NHRP packet     Types in the range 128 to 255 are reserved for research or use in     other protocol development and will be administered by IANA as     described inSection 9.   ar$shtl     Type & length of source NBMA address interpreted in the context of     the 'address family number'[6] indicated by ar$afn.  See below for     more details.   ar$sstl     Type & length of source NBMA subaddress interpreted in the context     of the 'address family number'[6] indicated by ar$afn.  When an     NBMA technology has no concept of a subaddress, the subaddress     length is always coded ar$sstl = 0 and no storage is allocated for     the subaddress in the appropriate mandatory part.  See below for     more details.   Subnetwork layer address type/length fields (e.g., ar$shtl, Cli Addr   T/L) and subnetwork layer subaddresses type/length fields (e.g.,   ar$sstl, Cli SAddr T/L) are coded as follows:    7 6 5 4 3 2 1 0   +-+-+-+-+-+-+-+-+   |0|x|  length   |   +-+-+-+-+-+-+-+-+   The most significant bit is reserved and MUST be set to zero. The   second most significant bit (x) is a flag indicating whether the   address being referred to is in:      - NSAP format (x = 0).      - Native E.164 format (x = 1).Luciani, et. al.            Standards Track                    [Page 15]

RFC 2332                       NBMA NHRP                      April 1998   For NBMA technologies that use neither NSAP nor E.164 format   addresses, x = 0 SHALL be used to indicate the native form for the   particular NBMA technology.   If the NBMA network is ATM and a subaddress (e.g., Source NBMA   SubAddress, Client NBMA SubAddress) is to be included in any part of   the NHRP packet then ar$afn MUST be set to 0x000F; further, the   subnetwork layer address type/length fields (e.g., ar$shtl, Cli Addr   T/L) and subnetwork layer subaddress type/length fields (e.g.,   ar$sstl, Cli SAddr T/L) MUST be coded as in [11].  If the NBMA   network is ATM and no subaddress field is to be included in any part   of the NHRP packet then ar$afn MAY be set to 0x0003 (NSAP) or 0x0008   (E.164) accordingly.   The bottom 6 bits is an unsigned integer value indicating the length   of the associated NBMA address in octets. If this value is zero the   flag x is ignored.5.2.0 Mandatory Part   The Mandatory Part of the NHRP packet contains the operation specific   information (e.g., NHRP Resolution Request/Reply, etc.) and variable   length data which is pertinent to the packet type.5.2.0.1 Mandatory Part Format   Sections5.2.1 through5.2.6 have a very similar mandatory part.   This mandatory part includes a common header and zero or more Client   Information Entries (CIEs).Section 5.2.7 has a different format   which is specified in that section.   The common header looks like the following:    0                   1                   2                   3    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   | Src Proto Len | Dst Proto Len |           Flags               |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                         Request ID                            |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |            Source NBMA Address (variable length)              |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |          Source NBMA Subaddress (variable length)             |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |          Source Protocol Address (variable length)            |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |       Destination  Protocol Address (variable length)         |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+Luciani, et. al.            Standards Track                    [Page 16]

RFC 2332                       NBMA NHRP                      April 1998   And the CIEs have the following format:    0                   1                   2                   3    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |    Code       | Prefix Length |         unused                |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |  Maximum Transmission Unit    |        Holding Time           |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |  Cli Addr T/L | Cli SAddr T/L | Cli Proto Len |  Preference   |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |            Client NBMA Address (variable length)              |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |           Client NBMA Subaddress (variable length)            |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |          Client Protocol Address (variable length)            |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                        .....................   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |    Code       | Prefix Length |         unused                |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |  Maximum Transmission Unit    |        Holding Time           |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |  Cli Addr T/L | Cli SAddr T/L | Cli Proto Len |  Preference   |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |            Client NBMA Address (variable length)              |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |           Client NBMA Subaddress (variable length)            |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |          Client Protocol Address (variable length)            |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   The meanings of the fields are as follows:   Src Proto Len     This field holds the length in octets of the Source Protocol     Address.   Dst Proto Len     This field holds the length in octets of the Destination Protocol     Address.   Flags     These flags are specific to the given message type and they are     explained in each section.Luciani, et. al.            Standards Track                    [Page 17]

RFC 2332                       NBMA NHRP                      April 1998   Request ID     A value which, when coupled with the address of the source,     provides a unique identifier for the information contained in a     "request" packet.  This value is copied directly from an "request"     packet into the associated "reply".  When a sender of a "request"     receives "reply", it will compare the Request ID and source address     information in the received "reply" against that found in its     outstanding "request" list.  When a match is found then the     "request" is considered to be acknowledged.     The value is taken from a 32 bit counter that is incremented each     time a new "request" is transmitted.  The same value MUST be used     when resending a "request", i.e., when a "reply" has not been     received for a "request" and a retry is sent after an appropriate     interval.     It is RECOMMENDED that the initial value for this number be 0.  A     node MAY reuse a sequence number if and only if the reuse of the     sequence number is not precluded by use of a particular method of     synchronization (e.g., as described inAppendix A).   The NBMA address/subaddress form specified below allows combined   E.164/NSAPA form of NBMA addressing. For NBMA technologies without a   subaddress concept, the subaddress field is always ZERO length and   ar$sstl = 0.   Source NBMA Address     The Source NBMA address field is the address of the source station     which is sending the "request". If the field's length as specified     in ar$shtl is 0 then no storage is allocated for this address at     all.   Source NBMA SubAddress     The Source NBMA subaddress field is the address of the source     station which is sending the "request".  If the field's length as     specified in ar$sstl is 0 then no storage is allocated for this     address at all.   For those NBMA technologies which have a notion of "Calling Party   Addresses", the Source NBMA Addresses above are the addresses used   when signaling for an SVC.   "Requests" and "indications" follow the routed path from Source   Protocol Address to the Destination Protocol Address. "Replies", on   the other hand, follow the routed path from the Destination Protocol   Address back to the Source Protocol Address with the followingLuciani, et. al.            Standards Track                    [Page 18]

RFC 2332                       NBMA NHRP                      April 1998   exceptions: in the case of a NHRP Registration Reply and in the case   of an NHC initiated NHRP Purge Request, the packet is always returned   via a direct VC (see Sections5.2.4 and5.2.5).   Source Protocol Address     This is the protocol address of the station which is sending the     "request".  This is also the protocol address of the station toward     which a "reply" packet is sent.   Destination Protocol Address     This is the protocol address of the station toward which a     "request" packet is sent.   Code     This field is message specific.  See the relevant message sections     below.  In general, this field is a NAK code; i.e., when the field     is 0 in a reply then the packet is acknowledging a request and if     it contains any other value the packet contains a negative     acknowledgment.   Prefix Length     This field is message specific.  See the relevant message sections     below.  In general, however, this fields is used to indicate that     the information carried in an NHRP message pertains to an     equivalence class of internetwork layer addresses rather than just     a single internetwork layer address specified. All internetwork     layer addresses that match the first "Prefix Length" bit positions     for the specific internetwork layer address are included in the     equivalence class.  If this field is set to 0x00 then this field     MUST be ignored and no equivalence information is assumed (note     that 0x00 is thus equivalent to 0xFF).   Maximum Transmission Unit     This field gives the maximum transmission unit for the relevant     client station.  If this value is 0 then either the default MTU is     used or the MTU negotiated via signaling is used if such     negotiation is possible for the given NBMA.   Holding Time     The Holding Time field specifies the number of seconds for which     the Next Hop NBMA information specified in the CIE is considered to     be valid.  Cached information SHALL be discarded when the holding     time expires.  This field must be set to 0 on a NAK.Luciani, et. al.            Standards Track                    [Page 19]

RFC 2332                       NBMA NHRP                      April 1998   Cli Addr T/L     Type & length of next hop NBMA address specified in the CIE.  This     field is interpreted in the context of the 'address family     number'[6] indicated by ar$afn (e.g., ar$afn=0x0003 for ATM).   Cli SAddr T/L     Type & length of next hop NBMA subaddress specified in the CIE.     This field is interpreted in the context of the 'address family     number'[6] indicated by ar$afn (e.g., ar$afn=0x0015 for ATM makes     the address an E.164 and the subaddress an ATM Forum NSAP address).     When an NBMA technology has no concept of a subaddress, the     subaddress is always null with a length of 0.  When the address     length is specified as 0 no storage is allocated for the address.   Cli Proto Len     This field holds the length in octets of the Client Protocol     Address specified in the CIE.   Preference     This field specifies the preference for use of the specific CIE     relative to other CIEs.  Higher values indicate higher preference.     Action taken when multiple CIEs have equal or highest preference     value is a local matter.   Client NBMA Address     This is the client's NBMA address.   Client NBMA SubAddress     This is the client's NBMA subaddress.   Client Protocol Address     This is the client's internetworking layer address specified.   Note that an NHS may cache source address binding information from an   NHRP Resolution Request if and only if the conditions described inSection 6.2 are met for the NHS.  In all other cases, source address   binding information appearing in an NHRP message MUST NOT be cached.5.2.1 NHRP Resolution Request   The NHRP Resolution Request packet has a Type code of 1. Its   mandatory part is coded as described inSection 5.2.0.1 and the   message specific meanings of the fields are as follows:   Flags - The flags field is coded as follows:Luciani, et. al.            Standards Track                    [Page 20]

RFC 2332                       NBMA NHRP                      April 1998      0                   1      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     |Q|A|D|U|S|       unused        |     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     Q       Set if the station sending the NHRP Resolution Request is a       router; clear if the it is a host.     A       This bit is set in a NHRP Resolution Request if only       authoritative next hop information is desired and is clear       otherwise.  See the NHRP Resolution Reply section below for       further details on the "A" bit and its usage.     D       Unused (clear on transmit)     U       This is the Uniqueness bit. This bit aids in duplicate address       detection.  When this bit is set in an NHRP Resolution Request       and one or more entries exist in the NHS cache which meet the       requirements of the NHRP Resolution Request then only the CIE in       the NHS's cache with this bit set will be returned.  Note that       even if this bit was set at registration time, there may still be       multiple CIEs that might fulfill the NHRP Resolution Request       because an entire subnet can be registered through use of the       Prefix Length in the CIE and the address of interest might be       within such a subnet. If the "uniqueness" bit is set and the       responding NHS has one or more cache entries which match the       request but no such cache entry has the "uniqueness" bit set,       then the NHRP Resolution Reply returns with a NAK code of "13 -       Binding Exists But Is Not Unique" and no CIE is included.  If a       client wishes  to  receive  non- unique  Next  Hop Entries, then       the client must have the "uniqueness" bit set to zero in its NHRP       Resolution Request. Note that when this bit is set in an NHRP       Registration Request, only a single CIE may be specified in the       NHRP Registration Request and that CIE must have the Prefix       Length field set to 0xFF.     S       Set if the binding between the Source Protocol Address and the       Source NBMA information in the NHRP Resolution Request is       guaranteed to be stable and accurate (e.g., these addresses are       those of an ingress router which is connected to an ethernet stub       network or the NHC is an NBMA attached host).Luciani, et. al.            Standards Track                    [Page 21]

RFC 2332                       NBMA NHRP                      April 1998   Zero or one CIEs (seeSection 5.2.0.1) may be specified in an NHRP   Resolution Request.  If one is specified then that entry carries the   pertinent information for the client sourcing the NHRP Resolution   Request.  Usage of the CIE in the NHRP Resolution Request is   described below:     Prefix Length       If a CIE is specified in the NHRP Resolution Request then the       Prefix Length field may be used to qualify the widest acceptable       prefix which may be used to satisfy the NHRP Resolution Request.       In the case of NHRP Resolution Request/Reply, the Prefix Length       specifies the equivalence class of addresses which match the       first "Prefix Length" bit positions of the Destination Protocol       Address.  If the "U" bit is set in the common header then this       field MUST be set to 0xFF.     Maximum Transmission Unit       This field gives the maximum transmission unit for the source       station.  A possible use of this field in the NHRP Resolution       Request packet is for the NHRP Resolution Requester to ask for a       target MTU.     Holding Time       The Holding Time specified in the one CIE permitted to be       included in an NHRP Resolution Request is the amount of time       which the source address binding information in the NHRP       Resolution Request is permitted to cached by transit and       responding NHSs.  Note that this field may only have a non-zero       value if the S bit is set.     All other fields in the CIE MUST be ignored and SHOULD be set to 0.   The Destination Protocol Address in the common header of the   Mandatory Part of this message contains the protocol address of the   station for which resolution is desired.  An NHC MUST send the NHRP   Resolution Request directly to one of its serving NHSs (seeSection 3   for more information).5.2.2 NHRP Resolution Reply   The NHRP Resolution Reply packet has a Type code of 2. CIEs   correspond to Next Hop Entries in an NHS's cache which match the   criteria in the NHRP Resolution Request.  Its mandatory part is coded   as described inSection 5.2.0.1.  The message specific meanings of   the fields are as follows:   Flags - The flags field is coded as follows:Luciani, et. al.            Standards Track                    [Page 22]

RFC 2332                       NBMA NHRP                      April 1998      0                   1      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     |Q|A|D|U|S|       unused        |     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     Q       Copied from the NHRP Resolution Request.  Set if the NHRP       Resolution Requester is a router;  clear if it is a host.     A       Set if the next hop CIE in the NHRP Resolution Reply is       authoritative; clear if the NHRP Resolution Reply is non-       authoritative.       When an NHS receives a NHRP Resolution Request for authoritative       information for which it is the authoritative source, it MUST       respond with a NHRP Resolution Reply containing all and only       those next hop CIEs which are contained in the NHS's cache which       both match the criteria of the NHRP Resolution Request and are       authoritative cache entries.  An NHS is an authoritative source       for a NHRP Resolution Request if the information in the NHS's       cache matches the NHRP Resolution Request criteria and that       information was obtained through a NHRP Registration Request or       through synchronization with an NHS which obtained this       information through a NHRP Registration Request.  An       authoritative cache entry is one which is obtained through a NHRP       Registration Request or through synchronization with an NHS which       obtained this information through a NHRP Registration Request.       An NHS obtains non-authoritative CIEs through promiscuous       listening to NHRP packets other than NHRP Registrations which are       directed at it.  A NHRP Resolution Request which indicates a       request for non-authoritative information should cause a NHRP       Resolution Reply which contains all entries in the replying NHS's       cache (i.e., both authoritative and non-authoritative) which       match the criteria specified in the request.     D       Set if the association between destination and the associate next       hop information included in all CIEs of the NHRP Resolution Reply       is guaranteed to be stable for the lifetime of the information       (the holding time).  This is the case if the Next Hop protocol       address in a CIE identifies the destination (though it may be       different in value than the Destination address if the       destination system has multiple addresses) or if the destination       is not connected directly to the NBMA subnetwork but the egress       router to that destination is guaranteed to be stable (such asLuciani, et. al.            Standards Track                    [Page 23]

RFC 2332                       NBMA NHRP                      April 1998       when the destination is immediately adjacent to the egress router       through a non-NBMA interface).     U       This is the Uniqueness bit. See the NHRP Resolution Request       section above for details.  When this bit is set, only one CIE is       included since only one unique binding should exist in an NHS's       cache.     S       Copied from NHRP Resolution Request message.   One or more CIEs are specified in the NHRP Resolution Reply. Each CIE   contains NHRP next hop information which the responding NHS has   cached and which matches the parameters specified in the NHRP   Resolution Request.  If no match is found by the NHS issuing the NHRP   Resolution Reply then a single CIE is enclosed with the a CIE Code   set appropriately (see below) and all other fields MUST be ignored   and SHOULD be set to 0.  In order to facilitate the use of NHRP by   minimal client implementations, the first CIE MUST contain the next   hop with the highest preference value so that such an implementation   need parse only a single CIE.     Code       If this field is set to zero then this packet contains a       positively acknowledged NHRP Resolution Reply.  If this field       contains any other value then this message contains an NHRP       Resolution Reply NAK which means that an appropriate       internetworking layer to NBMA address binding was not available       in the responding NHS's cache.  If NHRP Resolution Reply contains       a Client Information Entry with a NAK Code other than 0 then it       MUST NOT contain any other CIE.  Currently defined NAK Codes are       as follows:       4 - Administratively Prohibited         An NHS may refuse an NHRP Resolution Request attempt for         administrative reasons (due to policy constraints or routing         state).  If so, the NHS MUST send an NHRP Resolution Reply         which contains a NAK code of 4.       5 - Insufficient Resources         If an NHS cannot serve a station due to a lack of resources         (e.g., can't store sufficient information to send a purge if         routing changes), the NHS MUST reply with a NAKed NHRP         Resolution Reply which contains a NAK code of 5.Luciani, et. al.            Standards Track                    [Page 24]

RFC 2332                       NBMA NHRP                      April 1998       12 - No Internetworking Layer Address to NBMA Address Binding            Exists         This code states that there were absolutely no internetworking         layer address to NBMA address bindings found in the responding         NHS's cache.       13 - Binding Exists But Is Not Unique         This code states that there were one or more internetworking         layer address to NBMA address bindings found in the responding         NHS's cache, however none of them had the uniqueness bit set.     Prefix Length       In the case of NHRP Resolution Reply, the Prefix Length specifies       the equivalence class of addresses which match the first "Prefix       Length" bit positions of the Destination Protocol Address.     Holding Time       The Holding Time specified in a CIE of an NHRP Resolution Reply       is the amount of time remaining before the expiration of the       client information which is cached at the replying NHS.  It is       not the value which was registered by the client.     The remainder of the fields for the CIE for each next hop are     filled out as they were defined when the next hop was registered     with the responding NHS (or one of the responding NHS's     synchronized servers) via the NHRP Registration Request.   Load-splitting may be performed when more than one Client Information   Entry is returned to a requester when equal preference values are   specified.  Also, the alternative addresses may be used in case of   connectivity failure in the NBMA subnetwork (such as a failed call   attempt in connection-oriented NBMA subnetworks).   Any extensions present in the NHRP Resolution Request packet MUST be   present in the NHRP Resolution Reply even if the extension is non-   Compulsory.   If an unsolicited NHRP Resolution Reply packet is received, an Error   Indication of type Invalid NHRP Resolution Reply Received SHOULD be   sent in response.   When an NHS that serves a given NHC receives an NHRP Resolution Reply   destined for that NHC then the NHS must MUST send the NHRP Resolution   Reply directly to the NHC (seeSection 3).Luciani, et. al.            Standards Track                    [Page 25]

RFC 2332                       NBMA NHRP                      April 19985.2.3 NHRP Registration Request   The NHRP Registration Request is sent from a station to an NHS to   notify the NHS of the station's NBMA information.  It has a Type code   of 3. Each CIE corresponds to Next Hop information which is to be   cached at an NHS.  The mandatory part of an NHRP Registration Request   is coded as described inSection 5.2.0.1.  The message specific   meanings of the fields are as follows:   Flags - The flags field is coded as follows:      0                   1      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     |U|         unused              |     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     U       This is the Uniqueness bit. When set in an NHRP Registration       Request, this bit indicates that the registration of the protocol       address is unique within the confines of the set of synchronized       NHSs.  This "uniqueness" qualifier MUST be stored in the NHS/NHC       cache.  Any attempt to register a binding between the protocol       address and an NBMA address when this bit is set MUST be rejected       with a Code of "14 - Unique Internetworking Layer Address Already       Registered" if the replying NHS already has a cache entry for the       protocol address and the cache entry has the "uniqueness" bit       set.  A registration of a CIE's information is rejected when the       CIE is returned with the Code field set to anything other than       0x00.  See the description of the uniqueness bit in NHRP       Resolution Request section above for further details.  When this       bit is set only, only one CIE MAY be included in the NHRP       Registration Request.   Request ID     The request ID has the same meaning as described inSection5.2.0.1.  However, the request ID for NHRP Registrations which is     maintained at each client MUST be kept in non-volatile memory so     that when a client crashes and reregisters there will be no     inconsistency in the NHS's database.  In order to reduce the     overhead associated with updating non-volatile memory, the actual     updating need not be done with every increment of the Request ID     but could be done, for example, every 50 or 100 increments.  In     this scenario, when a client crashes and reregisters it knows to     add 100 to the value of the Request ID in the non-volatile memory     before using the Request ID for subsequent registrations.Luciani, et. al.            Standards Track                    [Page 26]

RFC 2332                       NBMA NHRP                      April 1998   One or more CIEs are specified in the NHRP Registration Request.   Each CIE contains next hop information which a client is attempting   to register with its servers.  Generally, all fields in CIEs enclosed   in NHRP Registration Requests are coded as described inSection5.2.0.1.  However, if a station is only registering itself with the   NHRP Registration Request then it MAY code the Cli Addr T/L, Cli   SAddr T/L, and Cli Proto Len as zero which signifies that the client   address information is to be taken from the source information in the   common header (seeSection 5.2.0.1).  Below, further clarification is   given for some fields in a CIE in the context of a NHRP Registration   Request.     Code       This field is set to 0x00 in NHRP Registration Requests.     Prefix Length       This field may be used in a NHRP Registration Request to register       equivalence information for the Client Protocol Address specified       in the CIE of an NHRP Registration Request In the case of NHRP       Registration Request, the Prefix Length specifies the equivalence       class of addresses which match the first "Prefix Length" bit       positions of the Client Protocol Address.  If the "U" bit is set       in the common header then this field MUST be set to 0xFF.   The NHRP Registration Request is used to register an NHC's NHRP   information with its NHSs.  If an NHC is configured with the protocol   address of a serving NHS then the NHC may place the NHS's protocol   address in the Destination Protocol Address field of the NHRP   Registration Request common header otherwise the NHC must place its   own protocol address in the Destination Protocol Address field.   When an NHS receives an NHRP Registration Request which has the   Destination Protocol Address field set to an address which belongs to   a LIS/LAG for which the NHS is serving then if the Destination   Protocol Address field is equal to the Source Protocol Address field   (which would happen if the NHC put its protocol address in the   Destination Protocol Address) or the Destination Protocol Address   field is equal to the protocol address of the NHS then the NHS   processes the NHRP Registration Request after doing appropriate error   checking (including any applicable policy checking).   When an NHS receives an NHRP Registration Request which has the   Destination Protocol Address field set to an address which does not   belong to a LIS/LAG for which the NHS is serving then the NHS   forwards the packet down the routed path toward the appropriate   LIS/LAG.Luciani, et. al.            Standards Track                    [Page 27]

RFC 2332                       NBMA NHRP                      April 1998   When an NHS receives an NHRP Registration Request which has the   Destination Protocol Address field set to an address which belongs to   a LIS/LAG for which the NHS is serving then if the Destination   Protocol Address field does not equal the Source Protocol Address   field and the Destination Protocol Address field does not equal the   protocol address of the NHS then the NHS forwards the message to the   appropriate NHS within the LIS/LAG as specified by Destination   Protocol Address field.   It is possible that a misconfigured station will attempt to register   with the wrong NHS (i.e., one that cannot serve it due to policy   constraints or routing state).  If this is the case, the NHS MUST   reply with a NAK-ed Registration Reply of type Can't Serve This   Address.   If an NHS cannot serve a station due to a lack of resources, the NHS   MUST reply with a NAK-ed Registration Reply of type Registration   Overflow.   In order to keep the registration entry from being discarded, the   station MUST re-send the NHRP Registration Request packet often   enough to refresh the registration, even in the face of occasional   packet loss. It is recommended that the NHRP Registration Request   packet be sent at an interval equal to one-third of the Holding Time   specified therein.5.2.4 NHRP Registration Reply   The NHRP Registration Reply is sent by an NHS to a client in response   to that client's NHRP Registration Request. If the Code field of a   CIE in the NHRP Registration Reply has anything other than zero in it   then the NHRP Registration Reply is a NAK otherwise the reply is an   ACK.  The NHRP Registration Reply has a Type code of 4.   An NHRP Registration Reply is formed from an NHRP Registration   Request by changing the type code to 4, updating the CIE Code field,   and filling in the appropriate extensions if they exist.  The message   specific meanings of the fields are as follows:   Attempts to register the information in the CIEs of an NHRP   Registration Request may fail for various reasons.  If this is the   case then each failed attempt to register the information in a CIE of   an NHRP Registration Request is logged in the associated NHRP   Registration Reply by setting the CIE Code field to the appropriate   error code as shown below:Luciani, et. al.            Standards Track                    [Page 28]

RFC 2332                       NBMA NHRP                      April 1998     CIE Code       0 - Successful Registration         The information in the CIE was successfully registered with the         NHS.       4 - Administratively Prohibited         An NHS may refuse an NHRP Registration Request attempt for         administrative reasons (due to policy constraints or routing         state).  If so, the NHS MUST send an NHRP Registration Reply         which contains a NAK code of 4.       5 - Insufficient Resources         If an NHS cannot serve a station due to a lack of resources,         the NHS MUST reply with a NAKed NHRP Registration Reply which         contains a NAK code of 5.       14 - Unique Internetworking Layer Address Already Registered         If a client tries to register a protocol address to NBMA         address binding with the uniqueness bit on and the protocol         address already exists in the NHS's cache then if that cache         entry also has the uniqueness bit on then this NAK Code is         returned in the CIE in the NHRP Registration Reply.   Due to the possible existence of asymmetric routing, an NHRP   Registration Reply may not be able to merely follow the routed path   back to the source protocol address specified in the common header of   the NHRP Registration Reply.  As a result, there MUST exist a direct   NBMA level connection between the NHC and its NHS on which to send   the NHRP Registration Reply before NHRP Registration Reply may be   returned to the NHC.  If such a connection does not exist then the   NHS must setup such a connection to the NHC by using the source NBMA   information supplied in the common header of the NHRP Registration   Request.5.2.5 NHRP Purge Request   The NHRP Purge Request packet is sent in order to invalidate cached   information in a station.  The NHRP Purge Request packet has a type   code of 5.  The mandatory part of an NHRP Purge Request is coded as   described inSection 5.2.0.1.  The message specific meanings of the   fields are as follows:   Flags - The flags field is coded as follows:Luciani, et. al.            Standards Track                    [Page 29]

RFC 2332                       NBMA NHRP                      April 1998      0                   1      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     |N|         unused              |     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     N       When set, this bit tells the receiver of the NHRP Purge Request       that the requester does not expect to receive an NHRP Purge       Reply.  If an unsolicited NHRP Purge Reply is received by a       station where that station is identified in the Source Protocol       Address of the packet then that packet must be ignored.   One or more CIEs are specified in the NHRP Purge Request.  Each CIE   contains next hop information which is to be purged from an NHS/NHC   cache.  Generally, all fields in CIEs enclosed in NHRP Purge Requests   are coded as described inSection 5.2.0.1.  Below, further   clarification is given for some fields in a CIE in the context of a   NHRP Purge Request.     Code       This field is set to 0x00 in NHRP Purge Requests.     Prefix Length       In the case of NHRP Purge Requests, the Prefix Length specifies       the equivalence class of addresses which match the first "Prefix       Length" bit positions of the Client Protocol Address specified in       the CIE.  All next hop information which contains a protocol       address which matches an element of this equivalence class is to       be purged from the receivers cache.     The Maximum Transmission Unit and Preference fields of the CIE are     coded as zero.  The Holding Time should be coded as zero but there     may be some utility in supplying a "short" holding time to be     applied to the matching next hop information before that     information would be purged; this usage is for further study. The     Client Protocol Address field and the Cli Proto Len field MUST be     filled in.  The Client Protocol Address is filled in with the     protocol address to be purged from the receiving station's cache     while the Cli Proto Len is set the length of the purged client's     protocol address.  All remaining fields in the CIE MAY be set to     zero although the client NBMA information (and associated length     fields) MAY be specified to narrow the scope of the NHRP Purge     Request if requester desires.  However, the receiver of an NHRP     Purge Request may choose to ignore the Client NBMA information if     it is supplied.Luciani, et. al.            Standards Track                    [Page 30]

RFC 2332                       NBMA NHRP                      April 1998   An NHRP Purge Request packet is sent from an NHS to a station to   cause it to delete previously cached information.  This is done when   the information may be no longer valid (typically when the NHS has   previously provided next hop information for a station that is not   directly connected to the NBMA subnetwork, and the egress point to   that station may have changed).   An NHRP Purge Request packet may also be sent from an NHC to an NHS   with which the NHC had previously registered.  This allows for an NHC   to invalidate its registration with NHRP before it would otherwise   expire via the holding timer. If an NHC does not have knowledge of a   protocol address of a serving NHS then the NHC must place its own   protocol address in the Destination Protocol Address field and   forward the packet along the routed path.  Otherwise, the NHC must   place the protocol address of a serving NHS in this field.   Serving NHSs may need to send one or more new NHRP Purge Requests as   a result of receiving a purge from one of their served NHCs since the   NHS may have previously responded to NHRP Resolution Requests for   that NHC's NBMA information.  These purges are "new" in that they are   sourced by the NHS and not the NHC;  that is, for each NHC that   previously sent a NHRP Resolution Request for the purged NHC NBMA   information, an NHRP Purge Request is sent which contains the Source   Protocol/NBMA Addresses of the NHS and the Destination Protocol   Address of the NHC which previously sent an NHRP Resolution Request   prior to the purge.   The station sending the NHRP Purge Request MAY periodically   retransmit the NHRP Purge Request until either NHRP Purge Request is   acknowledged or until the holding time of the information being   purged has expired. Retransmission strategies for NHRP Purge Requests   are a local matter.   When a station receives an NHRP Purge Request, it MUST discard any   previously cached information that matches the information in the   CIEs.   An NHRP Purge Reply MUST be returned for the NHRP Purge Request even   if the station does not have a matching cache entry assuming that the   "N" bit is off in the NHRP Purge Request.   If the station wishes to reestablish communication with the   destination shortly after receiving an NHRP Purge Request, it should   make an authoritative NHRP Resolution Request in order to avoid any   stale cache entries that might be present in intermediate NHSs (Seesection 6.2.2.).  It is recommended that authoritative NHRP   Resolution Requests be made for the duration of the holding time of   the old information.Luciani, et. al.            Standards Track                    [Page 31]

RFC 2332                       NBMA NHRP                      April 19985.2.6 NHRP Purge Reply   The NHRP Purge Reply packet is sent in order to assure the sender of   an NHRP Purge Request that all cached information of the specified   type has been purged from the station sending the reply.  The NHRP   Purge Reply has a type code of 6.   An NHRP Purge Reply is formed from an NHRP Purge Request by merely   changing the type code in the request to 6.  The packet is then   returned to the requester after filling in the appropriate extensions   if they exist.5.2.7  NHRP Error Indication   The NHRP Error Indication is used to convey error indications to the   sender of an NHRP packet.  It has a type code of 7.  The Mandatory   Part has the following format:    0                   1                   2                   3    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   | Src Proto Len | Dst Proto Len |            unused             |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |           Error Code          |        Error Offset           |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |            Source NBMA Address (variable length)              |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |          Source NBMA Subaddress (variable length)             |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |          Source Protocol Address (variable length)            |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |       Destination  Protocol Address (variable length)         |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |       Contents of NHRP Packet in error (variable length)      |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Src Proto Len     This field holds the length in octets of the Source Protocol     Address.   Dst Proto Len     This field holds the length in octets of the Destination Protocol     Address.Luciani, et. al.            Standards Track                    [Page 32]

RFC 2332                       NBMA NHRP                      April 1998   Error Code     An error code indicating the type of error detected, chosen from     the following list:       1 - Unrecognized Extension         When the Compulsory bit of an extension in NHRP packet is set,         the NHRP packet cannot be processed unless the extension has         been processed.  The responder MUST return an NHRP Error         Indication of type Unrecognized Extension if it is incapable of         processing the extension.  However, if a transit NHS (one which         is not going to generate a reply) detects an unrecognized         extension, it SHALL ignore the extension.       3 - NHRP Loop Detected         A Loop Detected error is generated when it is determined that         an NHRP packet is being forwarded in a loop.       6 - Protocol Address Unreachable         This error occurs when a packet it moving along the routed path         and it reaches a point such that the protocol address of         interest is not reachable.       7 - Protocol Error         A generic packet processing error has occurred (e.g., invalid         version number, invalid protocol type, failed checksum, etc.)       8 - NHRP SDU Size Exceeded         If the SDU size of the NHRP packet exceeds the MTU size of the         NBMA network then this error is returned.       9 - Invalid Extension         If an NHS finds an extension in a packet which is inappropriate         for the packet type, an error is sent back to the sender with         Invalid Extension as the code.       10 - Invalid NHRP Resolution Reply Received         If a client receives a NHRP Resolution Reply for a Next Hop         Resolution Request which it believes it did not make then an         error packet is sent to the station making the reply with an         error code of Invalid Reply Received.Luciani, et. al.            Standards Track                    [Page 33]

RFC 2332                       NBMA NHRP                      April 1998       11 - Authentication Failure         If a received packet fails an authentication test then this         error is returned.       15 - Hop Count Exceeded         The hop count which was specified in the Fixed Header of an         NHRP message has been exceeded.   Error Offset     The offset in octets into the original NHRP packet in which an     error was detected.  This offset is calculated starting from the     NHRP Fixed Header.   Source NBMA Address     The Source NBMA address field is the address of the station which     observed the error.   Source NBMA SubAddress     The Source NBMA subaddress field is the address of the station     which observed the error.  If the field's length as specified in     ar$sstl is 0 then no storage is allocated for this address at all.   Source Protocol Address     This is the protocol address of the station which issued the Error     packet.   Destination Protocol Address     This is the protocol address of the station which sent the packet     which was found to be in error.   An NHRP Error Indication packet SHALL NEVER be generated in response   to another NHRP Error Indication packet.  When an NHRP Error   Indication packet is generated, the offending NHRP packet SHALL be   discarded.  In no case should more than one NHRP Error Indication   packet be generated for a single NHRP packet.   If an NHS sees its own Protocol and NBMA Addresses in the Source NBMA   and Source Protocol address fields of a transiting NHRP Error   Indication packet then the NHS will quietly drop the packet and do   nothing (this scenario would occur when the NHRP Error Indication   packet was itself in a loop).   Note that no extensions may be added to an NHRP Error Indication.Luciani, et. al.            Standards Track                    [Page 34]

RFC 2332                       NBMA NHRP                      April 19985.3  Extensions Part   The Extensions Part, if present, carries one or more extensions in   {Type, Length, Value} triplets.   Extensions have the following format:    0                   1                   2                   3    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |C|u|        Type               |        Length                 |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                         Value...                              |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   C     "Compulsory."  If clear, and the NHS does not recognize the type     code, the extension may safely be ignored.  If set, and the NHS     does not recognize the type code, the NHRP "request" is considered     to be in error.  (See below for details.)   u     Unused and must be set to zero.   Type     The extension type code (see below).  The extension type is not     qualified by the Compulsory bit, but is orthogonal to it.   Length     The length in octets of the value (not including the Type and     Length fields;  a null extension will have only an extension header     and a length of zero).   When extensions exist, the extensions list is terminated by the Null   TLV, having Type = 0 and Length = 0.   Extensions may occur in any order, but any particular extension type   may occur only once in an NHRP packet unless explicitly stated to the   contrary in the extensions definition.  For example, the vendor-   private extension may occur multiple times in a packet in order to   allow for extensions which do not share the same vendor ID to be   represented.  It is RECOMMENDED that a given vendor include no more   than one Vendor Private Extension.   An NHS MUST NOT change the order of extensions.  That is, the order   of extensions placed in an NHRP packet by an NHC (or by an NHS when   an NHS sources a packet) MUST be preserved as the packet moves   between NHSs.  Minimal NHC implementations MUST only recognize, butLuciani, et. al.            Standards Track                    [Page 35]

RFC 2332                       NBMA NHRP                      April 1998   not necessarily parse, the Vendor Private extension and the End Of   Extensions extension.  Extensions are only present in a "reply" if   they were present in the corresponding "request" with the exception   of Vendor Private extensions.  The previous statement is not intended   to preclude the creation of NHS-only extensions which might be added   to and removed from NHRP packets by the same NHS; such extensions   MUST not be propagated to NHCs.   The Compulsory bit provides for a means to add to the extension set.   If the bit is set in an extension then the station responding to the   NHRP message which contains that extension MUST be able to understand   the extension (in this case, the station responding to the message is   the station that would issue an NHRP reply in response to a NHRP   request).  As a result, the responder MUST return an NHRP Error   Indication of type Unrecognized Extension.  If the Compulsory bit is   clear then the extension can be safely ignored; however, if an   ignored extension is in a "request" then it MUST be returned,   unchanged, in the corresponding "reply" packet type.   If a transit NHS (one which is not going to generate a "reply")   detects an unrecognized extension, it SHALL ignore the extension.  If   the Compulsory bit is set, the transit NHS MUST NOT cache the   information contained in the packet and MUST NOT identify itself as   an egress router (in the Forward Record or Reverse Record   extensions).  Effectively, this means, if a transit NHS encounters an   extension which it cannot process and which has the Compulsory bit   set then that NHS MUST NOT participate in any way in the protocol   exchange other than acting as a forwarding agent.   The NHRP extension Type space is subdivided to encourage use outside   the IETF.     0x0000 - 0x0FFF         Reserved for NHRP.     0x1000 - 0x11FF         Allocated to the ATM Forum.     0x1200 - 0x37FF         Reserved for the IETF.     0x3800 - 0x3FFF         Experimental use.   IANA will administer the ranges reserved for the IETF as described inSection 9. Values in the 'Experimental use' range have only local   significance.5.3.0  The End Of Extensions    Compulsory = 1    Type = 0    Length = 0Luciani, et. al.            Standards Track                    [Page 36]

RFC 2332                       NBMA NHRP                      April 1998   When extensions exist, the extensions list is terminated by the End   Of Extensions/Null TLV.5.3.1  Responder Address Extension    Compulsory = 1    Type = 3    Length = variable   This extension is used to determine the address of the NHRP   responder; i.e., the entity that generates the appropriate "reply"   packet for a given "request" packet.  In the case of an NHRP   Resolution Request, the station responding may be different (in the   case of cached replies) than the system identified in the Next Hop   field of the NHRP Resolution Reply.  Further, this extension may aid   in detecting loops in the NHRP forwarding path.   This extension uses a single CIE with the extension specific meanings   of the fields set as follows:   The Prefix Length fields MUST be set to 0 and ignored.   CIE Code     5 - Insufficient Resources       If the responder to an NHRP Resolution Request is an egress point       for the target of the address resolution request (i.e., it is one       of the stations identified in the list of CIEs in an NHRP       Resolution Reply) and the Responder Address extension is included       in the NHRP Resolution Request and insufficient resources to       setup a cut-through VC exist at the responder then the Code field       of the Responder Address Extension is set to 5 in order to tell       the client that a VC setup attempt would in all likelihood be       rejected; otherwise this field MUST be coded as a zero.  NHCs MAY       use this field to influence whether they attempt to setup a cut-       through to the egress router.   Maximum Transmission Unit     This field gives the maximum transmission unit preferred by the     responder.  If this value is 0 then either the default MTU is used     or the MTU negotiated via signaling is used if such negotiation is     possible for the given NBMA.   Holding Time     The Holding Time field specifies the number of seconds for which     the NBMA information of the responser is considered to be valid.     Cached information SHALL be discarded when the holding time     expires.Luciani, et. al.            Standards Track                    [Page 37]

RFC 2332                       NBMA NHRP                      April 1998   "Client Address" information is actually "Responder Address"   information for this extension.  Thus, for example, Cli Addr T/L is   the responder NBMA address type and length field.   If a "requester" desires this information, the "requester" SHALL   include this extension with a value of zero.  Note that this implies   that no storage is allocated for the Holding Time and Type/Length   fields until the "Value" portion of the extension is filled out.   If an NHS is generating a "reply" packet in response to a "request"   containing this extension, the NHS SHALL include this extension,   containing its protocol address in the "reply".  If an NHS has more   than one protocol address, it SHALL use the same protocol address   consistently in all of the Responder Address, Forward Transit NHS   Record, and Reverse Transit NHS Record extensions.  The choice of   which of several protocol address to include in this extension is a   local matter.   If an NHRP Resolution Reply packet being forwarded by an NHS contains   a protocol address of that NHS in the Responder Address Extension   then that NHS SHALL generate an NHRP Error Indication of type "NHRP   Loop Detected" and discard the NHRP Resolution Reply.   If an NHRP Resolution Reply packet is being returned by an   intermediate NHS based on cached data, it SHALL place its own address   in this extension (differentiating it from the address in the Next   Hop field).5.3.2  NHRP Forward Transit NHS Record Extension    Compulsory = 1    Type = 4    Length = variable   The NHRP Forward Transit NHS record contains a list of transit NHSs   through which a "request" has traversed.  Each NHS SHALL append to   the extension a Forward Transit NHS element (as specified below)   containing its Protocol address.  The extension length field and the   ar$chksum fields SHALL be adjusted appropriately.   The responding NHS, as described inSection 5.3.1, SHALL NOT update   this extension.   In addition, NHSs that are willing to act as egress routers for   packets from the source to the destination SHALL include information   about their NBMA Address.Luciani, et. al.            Standards Track                    [Page 38]

RFC 2332                       NBMA NHRP                      April 1998   This extension uses a single CIE per NHS Record element with the   extension specific meanings of the fields set as follows:   The Prefix Length fields MUST be set to 0 and ignored.   CIE Code     5 - Insufficient Resources       If an NHRP Resolution Request contains an NHRP Forward Transit       NHS Record Extension and insufficient resources to setup a cut-       through VC exist at the current transit NHS then the CIE Code       field for NHRP Forward Transit NHS Record Extension is set to 5       in order to tell the client that a VC setup attempt would in all       likelihood be rejected; otherwise this field MUST be coded as a       zero.  NHCs MAY use this field to influence whether they attempt       to setup a cut-through as described inSection 2.2.  Note that       the NHRP Reverse Transit NHS Record Extension MUST always have       this field set to zero.   Maximum Transmission Unit     This field gives the maximum transmission unit preferred by the     transit NHS.  If this value is 0 then either the default MTU is     used or the MTU negotiated via signaling is used if such     negotiation is possible for the given NBMA.   Holding Time     The Holding Time field specifies the number of seconds for which     the NBMA information of the transit NHS is considered to be valid.     Cached information SHALL be discarded when the holding time     expires.   "Client Address" information is actually "Forward Transit NHS   Address" information for this extension.  Thus, for example, Cli Addr   T/L is the transit NHS NBMA address type and length field.   If a "requester" wishes to obtain this information, it SHALL include   this extension with a length of zero.  Note that this implies that no   storage is allocated for the Holding Time and Type/Length fields   until the "Value" portion of the extension is filled out.   If an NHS has more than one Protocol address, it SHALL use the same   Protocol address consistently in all of the Responder Address,   Forward NHS Record, and Reverse NHS Record extensions.  The choice of   which of several Protocol addresses to include in this extension is a   local matter.Luciani, et. al.            Standards Track                    [Page 39]

RFC 2332                       NBMA NHRP                      April 1998   If a "request" that is being forwarded by an NHS contains the   Protocol Address of that NHS in one of the Forward Transit NHS   elements then the NHS SHALL generate an NHRP Error Indication of type   "NHRP Loop Detected" and discard the "request".5.3.3  NHRP Reverse Transit NHS Record Extension    Compulsory = 1    Type = 5    Length = variable   The NHRP Reverse Transit NHS record contains a list of transit NHSs   through which a "reply" has traversed.  Each NHS SHALL append a   Reverse Transit NHS element (as specified below) containing its   Protocol address to this extension.  The extension length field and   ar$chksum SHALL be adjusted appropriately.   The responding NHS, as described inSection 5.3.1, SHALL NOT update   this extension.   In addition, NHSs that are willing to act as egress routers for   packets from the source to the destination SHALL include information   about their NBMA Address.   This extension uses a single CIE per NHS Record element with the   extension specific meanings of the fields set as follows:   The CIE Code and Prefix Length fields MUST be set to 0 and ignored.   Maximum Transmission Unit     This field gives the maximum transmission unit preferred by the     transit NHS.  If this value is 0 then either the default MTU is     used or the MTU negotiated via signaling is used if such     negotiation is possible for the given NBMA.   Holding Time     The Holding Time field specifies the number of seconds for which     the NBMA information of the transit NHS is considered to be valid.     Cached information SHALL be discarded when the holding time     expires.   "Client Address" information is actually "Reverse Transit NHS   Address" information for this extension.  Thus, for example, Cli Addr   T/L is the transit NHS NBMA address type and length field.Luciani, et. al.            Standards Track                    [Page 40]

RFC 2332                       NBMA NHRP                      April 1998   If a "requester" wishes to obtain this information, it SHALL include   this extension with a length of zero.  Note that this implies that no   storage is allocated for the Holding Time and Type/Length fields   until the "Value" portion of the extension is filled out.   If an NHS has more than one Protocol address, it SHALL use the same   Protocol address consistently in all of the Responder Address,   Forward NHS Record, and Reverse NHS Record extensions.  The choice of   which of several Protocol addresses to include in this extension is a   local matter.   If a "reply" that is being forwarded by an NHS contains the Protocol   Address of that NHS in one of the Reverse Transit NHS elements then   the NHS SHALL generate an NHRP Error Indication of type "NHRP Loop   Detected" and discard the "reply".   Note that this information may be cached at intermediate NHSs;  if   so, the cached value SHALL be used when generating a reply.5.3.4 NHRP Authentication Extension   Compulsory = 1 Type = 7 Length = variable   The NHRP Authentication Extension is carried in NHRP packets to   convey authentication information between NHRP speakers.  The   Authentication Extension may be included in any NHRP "request" or   "reply" only.   The authentication is always done pairwise on an NHRP hop-by-hop   basis;  i.e., the authentication extension is regenerated at each   hop.  If a received packet fails the authentication test, the station   SHALL generate an Error Indication of type "Authentication Failure"   and discard the packet. Note that one possible authentication failure   is the lack of an Authentication Extension; the presence or absence   of the Authentication Extension is a local matter.5.3.4.1 Header Format   The authentication header has the following format:Luciani, et. al.            Standards Track                    [Page 41]

RFC 2332                       NBMA NHRP                      April 1998   0                   1                   2                   3   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |   Reserved                    | Security Parameter Index (SPI)|   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |               Src Addr...                                     |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                                                               |   +-+-+-+-+-+-+-+-+-+-+ Authentication Data... -+-+-+-+-+-+-+-+-+-+   |                                                               |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Security Parameter Index (SPI) can be thought of as an index into a   table that maintains the keys and other information such as hash   algorithm. Src and Dst communicate either offline using manual keying   or online using a key management protocol to populate this table. The   sending NHRP entity always allocates the SPI and the parameters   associated with it.   Src Addr a variable length field is the address assigned to the   outgoing interface. The length of the addr is obtained from the   source protocol length field in the mandatory part of the NHRP   header.  The tuple <spi, src addr> uniquely identifies the key and   other parameters that are used in authentication.   The length of the authentication data field  is dependent on the hash   algorithm used. The data field contains the keyed hash calculated   over the entire NHRP payload. The authentication data field is zeroed   out before the hash is calculated.5.3.4.2 SPI and Security Parameters Negotiation   SPI's can be negotiated either manually or using an Internet Key   Management protocol. Manual keying MUST be supported. The following   parameters are associated with the tuple <SPI, src>- lifetime,   Algorithm, Key. Lifetime indicates the duration in seconds for which   the key is valid. In case of manual keying, this duration can be   infinite. Also, in order to better support manual keying, there may   be multiple tuples active at the same time (Dst being the same).   Algorithm specifies the hash algorithm agreed upon by the two   entities. HMAC-MD5-128 [16] is the default algorithm. Other   algorithms MAY be supported by defining new values. IANA will assign   the numbers to identify the algorithm being used as described inSection 9.   Any Internet standard key management protocol MAY so be used to   negotiate the SPI and parameters.Luciani, et. al.            Standards Track                    [Page 42]

RFC 2332                       NBMA NHRP                      April 19985.3.4.3 Message Processing   At the time of adding the authentication extension header, src looks   up in a table to fetch the SPI and the security parameters based on   the outgoing interface address. If there are no entries in the table   and if there is support for key management, the src initiates the key   management protocol to fetch the necessary parameters. The src   constructs the Authentication Extension payload and calculates the   hash by zeroing authentication data field. The result replaces in the   zeroed authentication data field. The src address field in the   payload is the IP address assigned to the outgoing interface.   If key management is not supported and authentication is mandatory,   the packet is dropped and this information is logged.   On the receiving end, dst fetches the parameters based on the SPI and   the ip address in the authentication extension payload. The   authentication data field is extracted before zeroing out to   calculate the hash. It computes the hash on the entire payload and if   the hash does not match, then an "abnormal event" has occurred.5.3.4.4 Security Considerations   It is important that the keys chosen are strong as the security of   the entire system depends on the keys being chosen properly and the   correct implementation of the algorithms.   The security is performed on a hop by hop basis. The data received   can be trusted only so much as one trusts all the entities in the   path traversed. A chain of trust is established amongst NHRP entities   in the path of the NHRP Message . If the security in an NHRP entity   is compromised, then security in the entire NHRP domain is   compromised.   Data integrity covers the entire NHRP payload. This guarantees that   the message was not modified and the source is authenticated as well.   If authentication extension is not used or if the security is   compromised, then NHRP entities are liable to both spoofing attacks,   active attacks and passive attacks.   There is no mechanism to encrypt the messages. It is assumed that a   standard layer 3 confidentiality mechanism will be used to encrypt   and decrypt messages.  It is recommended to use an Internet standard   key management protocol to negotiate the keys between the neighbors.   Transmitting the keys in clear text, if other methods of negotiation   is used, compromises the security completely.Luciani, et. al.            Standards Track                    [Page 43]

RFC 2332                       NBMA NHRP                      April 1998   Any NHS is susceptible to Denial of Service (DOS) attacks that cause   it to become overloaded, preventing legitimate packets from being   acted upon properly. A rogue host can send request and registration   packets to the first hop NHS. If the authentication option is not   used, the registration packet is forwarded along the routed path   requiring processing along each NHS. If the authentication option is   used, then only the first hop NHS is susceptible to DOS attacks   (i.e., unauthenticated packets will be dropped rather than forwarded   on). If security of any host is compromised (i.e., the keys it is   using to communicate with an NHS become known), then a rogue host can   send NHRP packets to the first hop NHS of the host whose keys were   compromised, which will then forward them along the routed path as in   the case of unauthenticated packets.  However, this attack requires   that the rogue host to have the same first hop NHS as that of the   compromised host. Finally, it should be noted that denial of service   attacks that cause routers on the routed path to expend resources   processing NHRP packets are also susceptable to attacks that flood   packets at the same destination as contained in an NHRP packet's   Destination Protocol Address field.5.3.5  NHRP Vendor-Private Extension    Compulsory = 0    Type = 8    Length = variable   The NHRP Vendor-Private Extension is carried in NHRP packets to   convey vendor-private information or NHRP extensions between NHRP   speakers.    0                   1                   2                   3    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                  Vendor ID                    |  Data....     |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Vendor ID     802 Vendor ID as assigned by the IEEE [6]   Data     The remaining octets after the Vendor ID in the payload are     vendor-dependent data.   This extension may be added to any "request" or "reply" packet and it   is the only extension that may be included multiple times.  If the   receiver does not handle this extension, or does not match the VendorLuciani, et. al.            Standards Track                    [Page 44]

RFC 2332                       NBMA NHRP                      April 1998   ID in the extension then the extension may be completely ignored by   the receiver.  If a Vendor Private Extension is included in a   "request" then it must be copied to the corresponding "reply".6. Protocol Operation   In this section, we discuss certain operational considerations of   NHRP.6.1 Router-to-Router Operation   In practice, the initiating and responding stations may be either   hosts or routers.  However, there is a possibility under certain   conditions that a stable routing loop may occur if NHRP is used   between two routers.  In particular, attempting to establish an NHRP   path across a boundary where information used in route selection is   lost may result in a routing loop.  Such situations include the loss   of BGP path vector information, the interworking of multiple routing   protocols with dissimilar metrics (e.g, RIP and OSPF), etc.  In such   circumstances, NHRP should not be used.  This situation can be   avoided if there are no "back door" paths between the entry and   egress router outside of the NBMA subnetwork.  Protocol mechanisms to   relax these restrictions are under investigation.   In general it is preferable to use mechanisms, if they exist, in   routing protocols to resolve the egress point when the destination   lies outside of the NBMA subnetwork, since such mechanisms will be   more tightly coupled to the state of the routing system and will   probably be less likely to create loops.6.2 Cache Management Issues   The management of NHRP caches in the source station, the NHS serving   the destination, and any intermediate NHSs is dependent on a number   of factors.6.2.1 Caching Requirements   Source Stations     Source stations MUST cache all received NHRP Resolution Replies     that they are actively using.  They also must cache "incomplete"     entries, i.e., those for which a NHRP Resolution Request has been     sent but those for which an NHRP Resolution Reply has not been     received.  This is necessary in order to preserve the Request IDLuciani, et. al.            Standards Track                    [Page 45]

RFC 2332                       NBMA NHRP                      April 1998     for retries, and provides the state necessary to avoid triggering     NHRP Resolution Requests for every data packet sent to the     destination.     Source stations MUST purge expired information from their caches.     Source stations MUST purge the appropriate cached information upon     receipt of an NHRP Purge Request packet.     When a station has a co-resident NHC and NHS, the co-resident NHS     may reply to NHRP Resolution Requests from the co-resident NHC with     information which the station cached as a result of the co-resident     NHC making its own NHRP Resolution Requests as long as the co-     resident NHS follows the rules for Transit NHSs as seen below.   Serving NHSs     The NHS serving the destination (the one which responds     authoritatively to NHRP Resolution Requests) SHOULD cache protocol     address information from all NHRP Resolution Requests to which it     has responded if the information in the NHRP Resolution Reply has     the possibility of changing during its lifetime (so that an NHRP     Purge Request packet can be issued). The internetworking to NBMA     binding information provided by the source station in the NHRP     Resolution Request may also be cached if and only if the "S" bit is     set, the NHRP Resolution Request has included a CIE with the     Holding Time field set greater than zero (this is the valid Holding     Time for the source binding), and only for non-authoritative use     for a period not to exceed the Holding Time.   Transit NHSs     A Transit NHS (lying along the NHRP path between the source station     and the responding NHS) may cache source binding information     contained in NHRP Resolution Request packets that it forwards if     and only if the "S" bit is set, the NHRP Resolution Request has     included a CIE with the Holding Time field set greater than zero     (this is the valid Holding Time for the source binding), and only     for non-authoritative use for a period not to exceed the Holding     Time.     A Transit NHS may cache destination information contained in NHRP     Resolution Reply CIE if only if the D bit is set and then only for     non-authoritative use for a period not to exceed the Holding Time     value contained in the CIE.  A Transit NHS MUST NOT cache source     binding information contained in an NHRP Resolution Reply.Luciani, et. al.            Standards Track                    [Page 46]

RFC 2332                       NBMA NHRP                      April 1998     Further, a transit NHS MUST discard any cached information when the     prescribed time has expired.  It may return cached information in     response to non-authoritative NHRP Resolution Requests only.6.2.2 Dynamics of Cached Information   NBMA-Connected Destinations     NHRP's most basic function is that of simple NBMA address     resolution of stations directly attached to the NBMA subnetwork.     These mappings are typically very static, and appropriately chosen     holding times will minimize problems in the event that the NBMA     address of a station must be changed. Stale information will cause     a loss of connectivity, which may be used to trigger an     authoritative NHRP Resolution Request and bypass the old data.  In     the worst case, connectivity will fail until the cache entry times     out.     This applies equally to information marked in NHRP Resolution     Replies as being "stable" (via the "D" bit).   Destinations Off of the NBMA Subnetwork     If the source of an NHRP Resolution Request is a host and the     destination is not directly attached to the NBMA subnetwork, and     the route to that destination is not considered to be "stable," the     destination mapping may be very dynamic (except in the case of a     subnetwork where each destination is only singly homed to the NBMA     subnetwork).  As such the cached information may very likely become     stale.  The consequence of stale information in this case will be a     suboptimal path (unless the internetwork has partitioned or some     other routing failure has occurred).6.3 Use of the Prefix Length field of a CIE   A certain amount of care needs to be taken when using the Prefix   Length field of a CIE, in particular with regard to the prefix length   advertised (and thus the size of the equivalence class specified by   it).  Assuming that the routers on the NBMA subnetwork are exchanging   routing information, it should not be possible for an NHS to create a   black hole by advertising too large of a set of destinations, but   suboptimal routing (e.g., extra internetwork layer hops through the   NBMA) can result.  To avoid this situation an NHS that wants to send   the Prefix Length MUST obey the following rule:     The NHS examines the Network Layer Reachability Information (NLRI)     associated with the route that the NHS would use to forward towards     the destination (as specified by the Destination internetwork layerLuciani, et. al.            Standards Track                    [Page 47]

RFC 2332                       NBMA NHRP                      April 1998     address in the NHRP Resolution Request), and extracts from this     NLRI the shortest address prefix such that: (a) the Destination     internetwork layer address (from the NHRP Resolution Request) is     covered by the prefix, (b) the NHS does not have any routes with     NLRI which form a subset of what is covered by the prefix. The     prefix may then be used in the CIE.   The Prefix Length field of the CIE should be used with restraint, in   order to avoid NHRP stations choosing suboptimal transit paths when   overlapping prefixes are available.  This document specifies the use   of the prefix length only when all the destinations covered by the   prefix are "stable". That is, either:     (a) All destinations covered by the prefix are on the NBMA network,         or     (b) All destinations covered by the prefix are directly attached to         the NHRP responding station.   Use of the Prefix Length field of the CIE in other circumstances is   outside the scope of this document.6.4 Domino Effect   One could easily imagine a situation where a router, acting as an   ingress station to the NBMA subnetwork, receives a data packet, such   that this packet triggers an NHRP Resolution Request.  If the router   forwards this data packet without waiting for an NHRP transit path to   be established, then when the next router along the path receives the   packet, the next router may do exactly the same - originate its own   NHRP Resolution Request (as well as forward the packet).  In fact   such a data packet may trigger NHRP Resolution Request generation at   every router along the path through an NBMA subnetwork.  We refer to   this phenomena as the NHRP "domino" effect.   The NHRP domino effect is clearly undesirable.  At best it may result   in excessive NHRP traffic.  At worst it may result in an excessive   number of virtual circuits being established unnecessarily.   Therefore, it is important to take certain measures to avoid or   suppress this behavior.  NHRP implementations for NHSs MUST provide a   mechanism to address this problem. One possible strategy to address   this problem would be to configure a router in such a way that NHRP   Resolution Request generation by the router would be driven only by   the traffic the router receives over its non-NBMA interfaces   (interfaces that are not attached to an NBMA subnetwork).  Traffic   received by the router over its NBMA-attached interfaces would not   trigger NHRP Resolution Requests.  Such a router avoids the NHRP   domino effect through administrative means.Luciani, et. al.            Standards Track                    [Page 48]

RFC 2332                       NBMA NHRP                      April 19987. NHRP over Legacy BMA Networks   There would appear to be no significant impediment to running NHRP   over legacy broadcast subnetworks.  There may be issues around   running NHRP across multiple subnetworks. Running NHRP on broadcast   media has some interesting possibilities; especially when setting up   a cut-through for inter-ELAN inter-LIS/LAG traffic when one or both   end stations are legacy attached.  This use for NHRP requires further   research.8. Discussion   The result of an NHRP Resolution Request depends on how routing is   configured among the NHSs of an NBMA subnetwork.  If the destination   station is directly connected to the NBMA subnetwork and the routed   path to it lies entirely within the NBMA subnetwork, the NHRP   Resolution Replies always return the NBMA address of the destination   station itself rather than the NBMA address of some egress router.   On the other hand, if the routed path exits the NBMA subnetwork, NHRP   will be unable to resolve the NBMA address of the destination, but   rather will return the address of the egress router.  For   destinations outside the NBMA subnetwork, egress routers and routers   in the other subnetworks should exchange routing information so that   the optimal egress router may be found.   In addition to NHSs, an NBMA station could also be associated with   one or more regular routers that could act as "connectionless   servers" for the station.  The station could then choose to resolve   the NBMA next hop or just send the packets to one of its   connectionless servers.  The latter option may be desirable if   communication with the destination is short-lived and/or doesn't   require much network resources.  The connectionless servers could, of   course, be physically integrated in the NHSs by augmenting them with   internetwork layer switching functionality.9. IANA Considerations   IANA will take advice from the Area Director appointed designated   subject matter expert, in order to assign numbers from the various   number spaces described herein.  In the event that the Area Director   appointed designated subject matter expert is unavailable, the   relevant IESG Area Director will appoint another expert.  Any and all   requests for value assignment within a given number space will be   accepted when the usage of the value assignment documented.  Possible   forms of documentantion include, but is not limited to, RFCs or the   product of another cooperative standards body (e.g., the MPOA and   LANE subworking group of the ATM Forum).Luciani, et. al.            Standards Track                    [Page 49]

RFC 2332                       NBMA NHRP                      April 1998References   [1] Heinanen, J., and R. Govindan, "NBMA Address Resolution Protocol   (NARP)",RFC 1735, December 1994.   [2] Plummer, D., "Address Resolution Protocol", STD 37,RFC 826,   November 1982.   [3] Laubach, M., and J. Halpern, "Classical IP and ARP over ATM",RFC2225, April 1998.   [4] Piscitello,, D., and J. Lawrence, "Transmission of IP datagrams   over the SMDS service",RFC 1209, March 1991.   [5] Protocol Identification in the Network Layer, ISO/IEC TR   9577:1990.   [6] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2,RFC 1700,   October 1994.   [7] Heinanen, J., "Multiprotocol Encapsulation over ATM Adaptation   Layer 5",RFC 1483, July 1993.   [8] Malis, A., Robinson, D., and R. Ullmann, "Multiprotocol   Interconnect on X.25 and ISDN in the Packet Mode",RFC 1356, August   1992.   [9] Bradley, T., Brown, C., and A. Malis, "Multiprotocol Interconnect   over Frame Relay",RFC 1490, July 1993.   [10] Rekhter, Y., and D. Kandlur, ""Local/Remote" Forwarding Decision   in Switched Data Link Subnetworks",RFC 1937, May 1996.   [11] Armitage, G., "Support for Multicast over UNI 3.0/3.1 based ATM   Networks",RFC 2022, November 1996.   [12] Luciani, J., Armitage, G., and J. Halpern, "Server Cache   Synchronization Protocol (SCSP) - NBMA",RFC 2334, April 1998.   [13] Rekhter, Y., "NHRP for Destinations off the NBMA Subnetwork",   Work In Progress.   [14] Luciani, J., et. al., "Classical IP and ARP over ATM to NHRP   Transition", Work In Progress.   [15] Bradner, S., "Key words for use in RFCs to Indicate Requirement   Levels",BCP 14,RFC 2119, March 1997.Luciani, et. al.            Standards Track                    [Page 50]

RFC 2332                       NBMA NHRP                      April 1998   [16] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed Hashing   for Message Authentication",RFC 2104, February 1997.Acknowledgments   We would like to thank (in no particular order) Thomas Narten of IBM   for his comments in the role of Internet AD, Juha Heinenan of Telecom   Finland and Ramesh Govidan of ISI for their work on NBMA ARP and the   original NHRP draft, which served as the basis for this work.   Russell Gardo of IBM, John Burnett of Adaptive, Dennis Ferguson of   ANS, Andre Fredette of Bay Networks, Joel Halpern of Newbridge, Paul   Francis of NTT, Tony Li, Bryan Gleeson, and Yakov Rekhter of cisco,   and Grenville Armitage of Bellcore should also be acknowledged for   comments and suggestions that improved this work substantially.  We   would also like to thank the members of the ION working group of the   IETF, whose review and discussion of this document have been   invaluable.Authors' Addresses   James V. Luciani                    Dave Katz   Bay Networks                        cisco Systems   3 Federal Street                    170 W. Tasman Dr.   Mail Stop: BL3-03                   San Jose, CA 95134 USA   Billerica, MA 01821                 Phone:  +1 408 526 8284   Phone:  +1 978 916 4734             EMail:  dkatz@cisco.com   EMail:  luciani@baynetworks.com   David Piscitello                    Bruce Cole   Core Competence                     Juniper Networks   1620 Tuckerstown Road               3260 Jay St.   Dresher, PA 19025 USA               Santa Clara, CA 95054   Phone:  +1 215 830 0692             Phone:  +1 408 327 1900   EMail: dave@corecom.com             EMail:  bcole@jnx.com   Naganand Doraswamy   Bay Networks, Inc.   3 Federal Street   Mail Stop: Bl3-03   Billerica, MA 01801   Phone:  +1 978 916 1323   EMail: naganand@baynetworks.comLuciani, et. al.            Standards Track                    [Page 51]

RFC 2332                       NBMA NHRP                      April 1998Full Copyright Statement   Copyright (C) The Internet Society (1998).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Luciani, et. al.            Standards Track                    [Page 52]